1. Field of the Invention
This invention relates generally to a concrete material and, more particularly, to lightweight polymer concrete material having good cryogenic, insulating and load bearing characteristics and which is highly resistant to water absorption.
2. Description of the Prior Art
Those skilled in the relevant art are aware that certain combustable fluids such as liquified natural gas (LNG) are stored in large vessels in or near populated areas, these vessels usually being surrounded by a containment such as a dike to contain the liquid in the event of a spill. It is recognized that, if a spill does occur, the liquid will boil upon contact with its surroundings and form a vapor at a temperature -260.degree. F. The vapor produced from the boiling gains additional heat from its surroundings and disperses downwind through mixing with ambient air to form a cloud. The gas vapor is typically at a concentration in the lower flammable range, which range is of the order of 5 to 15% vapor in air.
It is therefore important to limit the distance that the cloud will travel and since at approximately -170.degree. F., the vapor becomes lighter than air, rises and dissipates, the distance travelled is directly proportional to the amount of vapor produced because the smaller the amount of vapor produced, the sooner it will warm and rise. It will therefore be appreciated that if the boil-off rate can be retarded, the production of vapor will also be retarded. This can be achieved by reducing liquid contact with the warm surface between the containment vessel and the dike and with the surface of the dike itself as by applying an insulating layer to those surfaces.
Such an insulating layer must be able to be effective in cryogenic environments, able to tolerate crushing loads applied by vehicles driving on it, remain in place, be fireproof to reduce the danger of fire in the event of a small leak and ignition, resist adverse weather conditions, such as freezing, thawing and rainwater absorption and must be substantially maintenance free.
U.S. Pat. No. 3,930,590 discloses an installation for storing liquified gas and particularly LNG. The patentee has recognized that in the event of leakage of the gas caused by damage to vessels or lines, the cold liquified gas flows into the collecting space provided around the vessel and will absorb a large quantity of heat from the environment thus causing the liquid to evaporate and form a concentrated cloud of inflammable gas. The patent therefore discloses a heat insulating material applied to the surface of the ground at least in the collecting space and possibly the wall or dike defining the collecting space.
According to the teaching of the patent, the insulating material is a lightweight concrete material derived by modifying a normal, dense concrete. The desired insulating concrete may be obtained by the substitution of a hollow or cellular aggregate, which includes air in the mix, for the usual gravel or gravel and sand aggregates. In order to counter any tendency of the insulating material to absorb water, the surface of the insulating material is coated with a layer of material such as epoxy resin.
Thus, the patentee employs a portland cement concrete using lightweight aggregates on a substrate with a sealant between the substrate and the concrete layers and with a sealant layer on the top of the concrete. In such an arrangement, the sealant deteriorates with time and permits moisture to permeate into the concrete. Moreover, the cement itself has a water content which is sealed in. The water, whether it permeates into the concrete or is present in the mix, provides an added degree of thermal conductivity and therefore limits the efficiency of the system and also contributes to deleterious cryogenic effects during possible freeze-thaw cycles causing the concrete to spall thus requiring relatively frequent maintenance.
Other prior art known to Applicants is disclosed in U.S. Pat. Nos. 3,047,184, 3,791,164 and 3,844,122. As employed herein the terms "concrete" and "concrete material" define the same product.